The protozoan parasite Plasmodium is the causative agent of malaria, which remains one of the most prominent public health challenges in the world today. Plasmodium-specific antibody responses are important for protecting against subsequent reinfections in humans and mice. However, while mice are protected after a single infection, protective immunity is slow to develop in humans due to the requirement of repeated infections for the generation of protective antibodies. Our long-term goal is to determine how protective antibody responses are generated and maintained in mice after Plasmodium infection, so that we can utilize this information to understand why antibody-mediated immunity is slow to develop in humans. Plasmodium-specific memory B cells are generated after infection in mice and humans; however, surprisingly little information is available regarding their specificity, phenotype, origin and affinity for malarial antigens. Our preliminary studies indicate that there are layers of heterogeneity within the memory B cell pool after Plasmodium infection and we hypothesize that this heterogeneity in the memory B cell pool contributes to functional diversity in a secondary infection. We propose to (Aim 1) characterize heterogeneous populations of memory B cells after P. yoelii 17X infection in mice and determine their origin. We will then (Aim 2) determine the function of distinct subsets of memory B cells after secondary infection. Additionally, our preliminary studies have identified two populations of memory T cells that express markers associated with follicular helper T cells. We propose to (Aim 3) determine if these populations of memory T cells are capable of differentiating into functional follicular helpe T cells that can support Ab production in a secondary infection and whether they are required for protection after challenge. To accomplish these goals we have developed innovative tools to track parasite- specific B cell responses at the cellular level utilizing parasites engineered to express hen egg-white lysozyme (HEL). Using HEL-specific transgenic B cells and a novel magnetic-bead based enrichment technique we can monitor and track the fate of antigen-specific B cells after infection with HEL expressing parasites. These innovative tools and approaches will provide valuable insight into understanding how protective immunity against Plasmodium is generated, is maintained, and functions in a secondary immune response and will identify key components involved in this process.